In pressurized medium systems, actuators which perform rotary or linear motions, such as motors and cylinders, are utilized in a known manner. In controlling the actuators, valves are utilized for controlling the pressurized medium for example to control the direction of movement of the actuator or to keep it stationary. Different functions of the actuators are implemented by suitable couplings of different valves.
Several different couplings for controlling an actuator are implemented particularly in directional valves comprising a spool which is axially movable in a bore in the valve body and which, in its different positions, couples the different connections and ports of the valves in a desired way. The different channels of the valve are connected to the connections and ports. Typically, the different channels include a pressure port P, one or more work ports, such as a work port A and a work port B, and a tank port T. The actuator is coupled to one or more work ports by means of pipes or hoses.
The spool comprises guide edges which throttle the volume flow. The pressure difference prevailing across the guide edges of the spool, together with the cross-sectional area of the flow (the aperture of the valve), determine the quantity of pressurized medium flowing through the valve, that is, the volume flow from one channel to another. The cross-sectional area of the flow depends on the position of the spool in relation to the valve body and the channels. By the design of the guide edges of the spool, for example various grooves and bevels, it is possible to affect the way in which the cross-sectional area of the flow changes from one position of the spool to another.
The pressure in the work port of the valve may vary as the load of the actuator changes. In particular, there is often reason to limit a pressure increase, for example, to prevent breaking of the pressurized medium system and its components.
Conventionally, pressure control, for example restricting the pressure to a predetermined value, and the lowering of the pressure when it exceeds a predetermined value, is performed by one or more separate pressure valves connected to the control circuit to which the directional valve belongs. The pressure valve is placed, for example, in a line coupled to the work port or pressure port of the directional valve.
According to the prior art, the functions of pressure control and the directional valve can also be combined in the same valve. Publication EP-1184576-A1 discloses a directional valve which also comprises the necessary functional positions for pressure control.
However, the directional valve constructed according to the prior art and also comprising pressure control functions, contains a lot of separate component and is complex in view of manufacture and assembly.